Reality star Jonathan Cheban was roughed up by a thug who tried to steal his $500,000 watch after he posted an Instagram picture of it during lunch. The drama began when Kim Kardashian’s best friend was eating outside at Southampton favorite 75 Main, and posted a photo of his lobster lunch to announce to all his fans in the world he was there.

A gaggle of women arrived at his table asking to take pictures. Cheban obliged and then posted a shot of the Jacob & Co. diamond-encrusted Royal watch he was wearing with the somewhat boastful caption, “I think I overdressed my wrist today . . . 22 karats is a bit much for daytime LOL.”

That might not have been the best idea even in swanky Southampton.

Soon, “a guy came over and started asking some questions,” a spy said. “Jonathan was very friendly. But then the guy grabbed Jonathan . . . and tried to steal his half-million-dollar watch! Jonathan fought back to stop him, and he pushed Jonathan to the floor.”

A security guard then jumped into the fray. But the punk got away without the watch. A friend of Cheban told us he didn’t bother filing a police report because “He left the Hamptons. He was there for a private event. He was only [in town] for three hours. Disaster. He had to go to Europe . . . so he didn’t want to deal with [police] reports.” The source added, “He wasn’t badly hurt . . . his wrist got banged up.”